1. Field of the Invention
This invention relates to a process of drying and calcining bulk materials in a rotary kiln with countercurrent hot gases, wherein drying and any precalcining are effected in direct contact with hot gases in that portion of the rotary kiln which adjoins the feed end, calcining is effected by an indirect heat exchange over the length of an inserted body disposed before the discharge end of the rotary kiln, and hot gases are fed into the inserted body and flow from the latter into the rotary kiln.
2. Discussion of Prior Art
When bulk materials are to be dried and calcined, it is often necessary to effect the drying and any precalcining slowly and/or at lower temperatures whereas high temperatures are required for calcining.
German Pat. No. 261,997 discloses a drying drum in which the hot gases and the material to be treated are moved cocurrently. In the rotary kiln, a heating tube smaller in diameter is succeeded by a heating tube larger in diameter. The material to be treated is charged into the annulus. One part of the hot gases is passed into the annulus and the other part into the heating tube. The rotary kiln is arranged as a stationary housing and the exhaust gases are fed into said housing for a utilization of their heat and flow countercurrently through the housing before they are passed into the chimney.
In that process, the hottest gases contact the coldest material to be treated and in consideration of the gas temperature the rotary kiln and the heating tube must consist of heat-resistant steel. The exhaust gas may cool the rotary kiln.
German Offenlegungsschrift No. 14 33 860 discloses a process of making gypsum wherein a rotary kiln contains a second tube and the material to be treated travels in the annulus to the closed end of the rotary kiln and then enters the inner tube and travels through the latter in the opposite direction to the outlet of the inner tube. The hot gases flow through the inner tube and then through the annulus countercurrently to and in direct contact with the material to be treated. The direct contact in the high-temperature zone may give rise to hot spots under fluctuating operating conditions. The equipment is rather complicated and the long inner tube must consist of high-grade steel.
It is known from "Ullmann", 3rd edition, 1951, volume 1, page 597, that drying and calcining can be effected in a countercurrent drum dryer which contains a central tube for a subsequent admixing of dry air and in which the central tube extends over a major part of the rotary kiln as far as to the beginning of the drying zone. The hot gases flow partly through the annular space and partly through the central tube. In that arrangement the direct contact in the annulus may also result in hot spots and the central tube and the rotary kiln must be made of high-grade steel.
It is known that aluminum fluorine hydrates which have been predried in a flow tube dryer or a disc dryer can be calcined in a fluidized bed furnace or the calcination can be effected in a directly or indirectly heated rotary kiln or in an externally heated fluidized-bed furnace. In all cases the furnace must consist of high-grade steel and hot spots may form where a direct contact is effected.
It is known from French Pat No. 927,063 to preheat in direct contact with hot gases and to heat further by an indirect heat transfer. The rotary kiln contains a body which has been inserted from the discharge end of the kiln and which receives hot gases and delivers them to the interior of the rotary kiln. The inserted body consists of an inner tube and refractory brickwork between the inner tube and the shell of the rotary kiln. Passages near the wall of the rotary kiln are provided in that refractory brickwork in a starlike array and receive the material to be treated and discharge it at the discharge end of the rotary kiln. The disadvantages of that process reside in that the passages are very small in diameter so that the material to be treated can travel only at a low velocity and the throughput rate is low too. The conditions are even worse because the passages are not circular. Material is retained at the entrance to the passages. Owing to that retention of material and the low throughput rate through the passages, the rotary kiln can be operated only with a low bed height. Because the passages are filled to a high degree, the tumbling is poor; this results in a retention of heat with a possible formation of hot spots and crusts. The heat transfer from the hot gases to the material in the passages is poor and irregular.